1. Field of the Invention
The present invention relates to micro electromechanical (MEM) systems and, more particularly, to fabrication of MEM components with a high electrically isolated substrate.
2. Description of Related Art
Micro electromechanical (MEM) components are being progressively introduced into many electronic circuit applications and a variety of micro-sensor applications. Examples of MEM components are radio frequency (RF) switches, high Q capacitors, pressure transducers and accelerometers. One such MEM component, a MEM switch, is disclosed in U.S. Pat. No. 5,578,976 which issued to Rockwell International Corporation the assignee of the present application. This MEM switch is fabricated on a GaAs substrate with a cantilevered switch arm formed from silicon dioxide deposited upon a sacrificial layer. Contacts and electrodes are readily formed through deposition of gold and aluminum, respectively.
Another prior art method for creating cantilever beams required a deep anisotrophic etch into a silicon substrate and application of either a silicon nitride or oxide layer to coat the top and side walls of the exposed cut. An isotrophic etch of the silicon substrate undercuts and frees the MEM component. Unfortunately, this method is not readily adaptable for applications where non-conductive, high resistance substrates such as glass are desired for high isolation applications.
Another method, often referred to as a surface micro machining, uses a sacrificial layer such as silicon dioxide deposited on a silicon substrate. MEM component material, poly-silicon, by way of example, is then deposited, patterned and released. The poly-silicon layer is etched by a reactive ion etch to expose the sacrificial silicon dioxide layer. The sacrificial layer is then etched, usually with an acid (hydrofluoric acid), to release the MEM component. However, MEM components created from poly-silicon have limited mechanical strength and exhibit relatively poor electrical isolation. Further, production yields are poor using this method since the wet hydrofluoric etch often results in the MEM component sticking to the substrate rather than being suspended.
If a high electrical isolation is required, fabrication of MEM components on a glass substrate generally required either ionic (application of high voltage) or fusion (high temperature) bonding techniques to create MEM components. Both of these bonding techniques are poorly suited for use when semiconductor devices are present on the same substrate. Specifically, with ionic bonding the high voltage may damage sensitive electrical components while the high processing temperature associated with both ionic and fusion bonding may cause junctions depths to change affecting device performance and reliability. It is also known that such bonding techniques require very smooth surface to surface contact to ensure a good bond. If the surfaces do not mate within acceptable tolerances, the reaction or inter-diffusion process will result in a defective bond. Further, these bonding techniques are sensitive to surface contamination or irregularities which may result in bond failure sites or a decrease in production yields.
In another prior art method, a glass substrate is bonded to a silicon dioxide layer using ionic or fusion bonding techniques. Prior to bonding, the silicon dioxide layer is deposited on top of a silicon wafer so that the bonding process forms a glass-silicon dioxide-silicon composite structure. The silicon is patterned and wet etched to define the MEM component.
As mentioned above, ionic or fusion bonding require a high process temperature which are in the range of about 450xc2x0 C. to 500xc2x0 C. Further, the glass substrate must be conductive to facilitate bonding with the silicon dioxide. Such conductivity precludes achieving high electrical isolation in the final MEM system. Further still, with the wet etch used to release the MEM component, the structure often sticks to the substrate rather than remaining free standing.
The present invention provides a method that uses adhesive bonding to form a MEM component on top of a glass substrate so that the MEM component is electrically isolated from the substrates. Further, the present process uses a dry etch to release the MEM component. Thus, whatever the merits of the above described prior art methods, they do not achieve the benefits of the present invention.
The present invention relates to a fabrication process for manufacture of micro electromechanical (MEM) systems having components, such as cantilever supported beams, spaced above the substrate. This fabrication process uses as few as two lithographic masking steps depending on the complexity of the device and provides MEM components that are electrically isolated from the substrate.
Specifically, in one embodiment of the present invention, a composite silicon-film-glass substrate structure is formed. The silicon layer is processed by either polishing, grinding or etching to obtain the desired thickness, patterned to define the MEM component and etched to expose the film layer. The film layer is a sacrificial layer that is then patterned and dry etched to release the MEM component.
In other embodiment of the present invention, the process comprises the steps of growing a layer of doped silicon on a silicon wafer or substrate and depositing a layer of insulating material such as silicon dioxide on the doped silicon. This embodiment includes the use of a silicon on insulator (SOI) substrate. The silicon substrate is adhesive bonded to a glass substrate to create a composite silicon-silicon dioxide-silicon-adhesive-glass structure. The silicon is patterned and etched using anisotropic plasma dry etching techniques. A second patterning then follows to pattern the silicon dioxide layer and an oxygen plasma etch is performed to undercut the adhesive film and to release the doped silicon MEM component. This two-mask process provides single crystal silicon MEM component that is electrically isolated from the glass substrate but mechanically joined thereto.
The adhesive serves a dual role as a bonding agent and as a sacrificial layer that can be readily removed to release the MEM component in an effective and efficient manner. Specifically, the dry oxygen plasma etch undercuts the adhesive without causing the MEM component to stick to adjacent surfacesxe2x80x94a common problem with wet chemical releases. In addition the oxygen plasma is a benign process with respect to the other material of the composite structure.
In accordance with the present invention, fabrication of very small cantilever supported beams, switches or other micro electromechanical structures are readily incorporated with other circuit functions on an integrated circuit device. The present invention is particularly well suited for manufacture of long narrow freestanding beams parallel to the substrate that can move in response to pressure, electromagnetic, mechanical or other such stimuli.
Also, the surfaces of the substrates need not be perfectly smooth since the adhesive layer eliminates the need to use fusion or ionic bonding techniques to form the composite structure. Indeed, preexisting diffusions or surface irregularities may be present in the substrates with little or no impact on yield or the integrity of the composite structure. This provides very flexible design options since electrical components may be positioned in close proximity to the MEM component.
These and other advantages of the present invention not specifically described above will become clear within the detailed discussion herein.